Many emerging diseases of the late 20th and early 21st centuries are caused by pathogens that originally infected only animals. We usually explain this transfer of disease from animals to humans as the result of environmental changes, the invasion and destruction of animal habitats by humans that brings us into closer contact with the pathogen-carrying animals. To further understand this exchange of diseases, let us consider a very dangerous malaria that appears to be emerging in Malasia: Plasmodium knowlesi or “monkey malaria.”

In the 1960s, the National Institutes of Health established a laboratory in peninsular Malaysia for the very purpose of finding monkey malarias in humans. The researchers tested more than 1,000 people from the area and concluded that monkey malaria infections were not occurring in humans. They packed their bags and went home. Ironically, as soon as they arrived, one of them became seriously ill with a monkey malaria, proving that their original conclusion was mistaken, that monkey malarias could be passed to humans, although they rarely were.

Then, in 2004, a team of researchers from Malaysia surprised the medical community by reporting that a significant number of people on the island of Borneo were infected with a monkey malaria, Plasmodium knowlesi. They found that P. knowlesi infections accounted for as many as 80 percent of the severe cases of malaria in regional hospitals. Of even more concern, the team has shown that this malaria is a killer. The medical community is now asking whether P. knowlesi is a parasite that can be transmitted directly from human to human and if the outbreak has the potential to spread to the mainland and other regions of Southeast Asia.

What had occurred during the preceding 40 years that might account for this sudden discovery of monkey malaria in humans?

In the 1980s and 1990s, Malaysians had cut down their abundant rain forests for timber and replaced them with plantations that would produce an export product: Palm oil. Up to that time, most scientists reasoned, mosquitoes that carried monkey malaria had lived in the treetops, away from humans, and bitten only monkeys. Mosquitoes are particular about the blood they drink and undoubtedly preferred to bite monkeys. It is during this “blood feed” that malaria parasites are transmitted to their target.

The thinning of the rainforest and decline of the monkey population dramatically changed the mosquito’s options; they need blood to reproduce. People looking for employment came to the newly established plantations in number; hence, became an alternative source of blood for the mosquito that transmitted the monkey malaria. It would be easy to conclude that this is a central reason that infection with P. knowlesi in humans has taken a dramatic upsurge.

But is monkey malaria truly an emerging threat to human health, and is the tremendous increase in its diagnosis entirely the result of deforestation? Were there advances in medical technology during the decades between the 1960s and the early 20th Century that might equally account for its new prevalence?

Historically, doctors have diagnosed malaria infections by looking at a victim’s blood under the microscope and visually identifying the parasite. This is the “gold standard” for identification of a malaria infection and one used by the investigators in Malaysia in the 1960s. Unfortunately, Plasmodium knowlesi looks essentially identical to Plasmodium malaria, a parasite that causes malaria in humans.

It was only in 2004, the same year that the incidence of P. knowlesi appeared to increase so alarmingly, that scientists in Malaysia acquired the technology to analyze the DNA of malaria parasites. With DNA analysis they were for the first time able to distinguish P. knowlesi from P. malaria. Now, a new and rapid “onsite” diagnostic for P. knowlesi developed by McCutchan et al. (Emerg Infect Dis. 2008 November; 14(11): 1750–1752) is being used in Borneo to further accelerate the investigation. Although one cannot discount entirely the environmental changes in Malaysia as a cause for the increased incidence of P. knowlesi, it is likely that it had all along been causing acute malaria illness but had been mistaken for P. malaria.

It may be of interest to historians that Alfred Russell Wallace, who, along with Charles Darwin, was the cofounder of the “Theory of Evolution” (but never given equal credit for it), came up with the concept while he was suffering from the type of intense, daily fevers symptomatic of monkey malaria, on an island near Borneo with much the same environment as that of the epicenter of this “emerging disease.”